Ultrastructural and histochemical observations regarding the ovarian follicles of the amago salmon (Oncorhynchus rhodurus).

Pre- and postovulatory ovarian follicles of the amago salmon were investigated with special consideration given to the steroid production site. In the preovulatory specimens, the special thecal cells contained abundant mitochondria and smooth endoplasmic reticulum, as in the case of other steroid-producing cells, while the granulosa cells developed rough endoplasmic reticulum like protein-secreting cells. Remarkably ultrastructural changes occurred in the granulosa and the special thecal cells during the oocyte maturation stage; dilation of rough endoplasmic reticulum in the former cell and further development of smooth endoplasmic reticulum in the latter one. Histochemical reactions of delta 5-3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) activity were only positive in the special thecal cells of the postovulatory follicles. These results strongly suggest that the special thecal cells are the major sites of estrogen precursor synthesis in the amago salmon ovary.